An electroactive cobalt hexacyanoferrate (CoHCF) film was electrodeposited from a solution containing Co2+ and Fe(CN)(6)(3-) ions on the bare gold or the Au-colloid modified electrode. The cation (Li+, Na+, K+, Rb+, Cs+, Mg, Ca, Sr2+ and Ba2+) and the anion (F-, Cl- and Br-) effects on the redox peak of the CoHCF film were investigated in detail. On the other hand, the electrocatalytic oxidations of thiosulfate at the CoHCF/gold and CoHCF/Au-colloid/gold electrodes were compared. At the CoHCF/Au-colloid/gold electrode, we obtained a response current larger by a factor of 2 and a three times lower detection limit than those at a CoHCF/gold electrode. The linear ranges were 1.0 x 10(-4) to 2.8 x 10(-3) M for the CoHCF/gold electrode and 7.5 x 10(-5) to 4.8 x 10(-3) M for the CoHCF/Au-colloid/gold electrode. These results showed that the immobilized CoHCF at the Au-colloid modified electrode exhibited a higher catalytic activity and a wider linear range toward thiosulfate. Additionally, the effects of the applied potential and the solution pH were studied. (C) 2004 Elsevier B.V. All rights reserved.